It's not a tax on broadband, it's a tax on fixed phone lines to help pay for broadband in areas that don't have it.
Complete and utter bollocks if you ask me. I live in a town and already have broadband, why should I have to pay so those that live in rural / remote areas can get it? If they want it they can either move somewhere more practical or pay for it themselves.
You can make the same argument for rural roads/water supplies etc. And conversely, rural dwellers could object as to why they should pay more tax for the urban scum to get patched up after stabbing each other (NHS), burgling each other (Police) etc. The argument is that broadband is now as much a part of our national communications infrastructure as roads, telephones, TV etc and so if we as a nation want to have a decent comms infrastucture, then some serious investment needs to be made in laying down the foundations for that, so we can all benefit from it.